Find the radius of circle of illuminance, if a luminous object is placed at a distance `h` form the interface in denser medium.

Explanation

`tan C = (r)/(h), :. r = h tan C` But `C = sin^(-1) (1)/((mu_(d)//mu_(r)))`
So, `r = h tan [sin^(-1) (1)/((mu_(d)//mu_(r)))] = h. (mu_(r))/(sqrt(mu_(d)^(2)//mu_(r)^(2)))`